"We are actively recruiting egg donors!" reads the advertisement on the website of one of India's top in-vitro fertilisation clinics.

"Our patients are happy to pay generously for your generosity! They pay you up to rupees 40,000 (800 dollars) every time you donate".

A lack of regulation surrounding fertility services in India and the lucrative returns on offer to those that provide them has turned India into a popular hub of "IVF tourism".

Childless couples from overseas are attracted by the relatively low-cost treatment, as well as "friendly rules" when it comes to egg donors and surrogate motherhood.

According to the private Indian Society for Assisted Reproduction (ISAR), there are some 400 IVF clinics in the country, providing an estimated 30,000 assisted reproductive treatments a year.

There are no precise estimates for what percentage are taken up by foreigners, but doctors say overseas demand is fuelling a boom.

"Nearly half of our patients come from overseas. Of them, nearly half are of Indian origin," said Aniruddha Malpani, whose IVF clinic in Mumbai is considered among the country's best.

A full IVF cycle at the Malpani clinic costs 4,500 dollars, including medicines. In the United States, the average cost is 12,400 dollars, according to the American Society for Reproductive Medicine.

Malpani offers a top-end service, but the same treatment is available from other Indian clinics at less than half the price.

While cost is a big factor in drawing people from abroad, equally important is the lack of effective regulation.

"India has friendly rules. There are no restrictions on egg donation," said Manish Banker, vice president of the ISAR.

In Britain, the British Human Fertilisation and Embryology Authority (HFEA) has outlawed payments to surrogates and British IVF clinics allow doctors to implant only two embryos into the uterus in a treatment.